LEDs are semiconductor electronic devices capable of emitting light and composed by P- and N-type semiconductor materials. They can radiate light in the ultraviolet, visible, and infrared regions. Thanks to their advantages of saving power, long lifetime, and high brightness, in the recent trend of environmental protection, saving energy, and reducing carbon emission, the applications of LEDs, for example, traffic lights, streetlamps, flashlights, display devices, or illumination devices, become extensive increasingly.
Currently, most LED display devices, such as notebook computers or LCD panels, adopts LED driving circuits to output pulse width modulation (PWM) signals as the dimming signals of LEDs for adjusting the duty cycle of switching signals. Thereby, LEDs are switched on and off and thus achieving the purpose of adjusting the brightness of LEDs.
Nonetheless, in a general LED driving circuit, the inductance of the internal inductor in the driving circuit has to match the duty cycles of the switching signal and the output current to the LED. For example, if the input power supply, the output voltage, and the frequency of the switching signal are maintained, when the output current is lowered by increasing the duty cycle of the switching signal for tuning light, the inductance of the inductor should be larger for avoiding spikes in the switching signal and the output current.
Please refer to FIGS. 1A and 1B. FIG. 1A shows waveforms of the switching signal with 100% duty cycle of the driving circuit for LED according to the prior art; FIG. 1B shows waveforms of the switching signal with 20% duty cycle of the driving circuit for LED according to the prior art. As shown in the figures, when the duty cycle of the switching signal VGP is 100%, the output current IOP is normal. When the duty cycle of the switching signal VGP is 20%, because the inductance of the inductor inside the driving circuit is insufficient, spikes occur in both the switching signal VGP and the output current IOP. Thereby, for tuning LED light, a plurality of series inductors are usually disposed for meeting the requirement of larger inductance. Nonetheless, the plurality of series inductors will result in the problems of increased circuit area and cost.
Accordingly, the present invention provides a driving circuit for LED and a driving method thereof, which adjusts the switching frequency for supplying the required output current. Hence, the problems described above can be solved.